A Treatise On The Principles And Practice Of Dock Engineering

220 DOCK ENGINEERING. ing an intermittent into a continuons service. In the dredging of bars, the lowering of dock sills and floors, two of the main obstacles to the ideal condition can be artificially overcome, and the problem then simply résolves itself into a question of fixing a judicious limit to the expenditure incurred, so as to achieve the most beneficial result commensurate with the port’s resources and prospects. Docks in tideless seas, as the Mediterranean ;* in inland situations, as at Rouen and Bremen ; and in localities where there is only a small range of tide, as is tlie case at Glasgow and Southampton, are endowed by Nature with signal advantages in this respect, which enable them to dispense with all the costly apparatus necessary for periodically closing their entrances, together with all the time and labour involved in the operation, while, at the same time, it confers upon them special facilities for the prompt reception and discharge of shipping. On the other hand, such docks reproduce every fluctuation of the external water level, and from the very continuity of their systems, their entrances are liable to constitute quiescent depositing areas for silt and detritus, brought in by passing currents. This last drawback, however, is one from which tidal ports themselves are not altogether exempt. In determining the dispositions to be adopted for a dock entrance, the following points have primary importance, viz. :—(1) Site ; (2) direction ; and (3) size. Site._____The site should obviously be the most sheltered spot available for the purpose. Exposure during docking operations to the direct influ- ence of even a moderate gale may render a vessel temporarily nnmanage- able, and cause her to drift into situations dangerous alike to herself and to neighbouring craft. The writer has seen several lineal yards of granite coping at a dock entrance detruded by the stem of a vessel, under no way, but imperfectly controlled, while docking in a heavy swell. The strain upon entrance gates at such times is likewise exceedingly great, especially immediately after they liave been closed. Until sufficient head is acquired on the inner side, by the fall of the tide, to keep them fairly mitred, the leaves are undergoing a series of chafings and concussions against one another and the sill, and even when actual movement in them has ceased, they are still dynamically stressed by the impact of breaking waves. Bor this reason direct communication and by tliis is meant communi- cation in an uninterruptedly straight line—with the open sea is to be avoided, wherever practicable. In the case of ports on the seaboard, outer harbours or entrance channels should be provided of length, at least, sufficient to admit of a vessel losing the way which she may have gathered in making for her destination under stress of weather. The lengtli of sheltered reach necessary for tliis purpose will vaiy with particular circumstances, but the following instances may be cited as * The Mediterranean is not strietly tideless, but the range of tide is so small as to be negligible.